The present invention relates to a device and a method for electrical stimulation of the heart.
Experimental studies on animals and on man (such as those reported, for example, in the work xe2x80x9cElectronic inhibition and active facilitation of excitability in ventricular musclexe2x80x9d by J. M. Davidenko, M. Delnar, and J. Beaumont in J. Cardiov. Electrophysiol. Vol. 5, No. 11 November 1994, pages 945-960), have shown the effects of subthreshold electrical stimuli on response to subsequent stimulations. These effects are referred to briefly by the term electrotonic inhibition. According to this research, the introduction of a subthreshold conditioning stimulus between two above-threshold stimulations is followed by a transitory decay in the excitability of the muscle stimulated. In particular, a subthreshold stimulus triggered a certain period of time before the subsequent above-threshold stimulus may delay the response to the subsequent stimulus in question, increasing so-called latency, that is, the interval between the pulse and the respective rapid front of the action potential, and even having an actual inhibition effect. The degree of inhibition is directly proportional to the amplitude and to the duration of the subthreshold conditioning stimulus and is inversely proportional to the post-conditioning period between the electrotonic inhibition pulse and the next stimulation pulse.
Electrotonic inhibition can explain some phenomena which occur during atrial fibrillation in man. This applies in particular to the paradoxical effect of a reduction in ventricular frequency due to the action of a vagotonic drug such as digitalis (which reduces atrial refractory periods and therefore considerably increases the frequency of local atrial fibrillation) and to the effect observed, also during atrial fibrillation, of a reduction in the shortest R-R periods of the electrocardiograph signal, to the point of disappearance, when the ventricle is stimulated at longer intervals.
These phenomena can be explained only by electrotonic inhibition which would act on the atrioventricular node, actually reducing its conduction rate as a result of the increased atrial fibrillation frequency and because of the reverse conduction of ventricular stimuli.
In this connection, the following references may be consulted: xe2x80x9cOn the mechanism(s) of atrioventricular nodal transmission in atrial fibrillationxe2x80x9d by F. L. Meijler and J. Jalife in Cardiology 1997; 42(4); pages 375-384, and
xe2x80x9cAV nodal function during atrial fibrillation; the role of electrotonic modulation of propagationxe2x80x9d by F. L. Meijler, J. Jalife, J. Beaumont, and D. Vaidya in J. Cardiov. Electrophysiol. Vol. 7, No. 9 September 1996, pages 843-861.
Recent tests in which subthreshold stimuli were used to terminate re-entrant ventricular tachycardia, both at local level and by involving the atrioventricular node, with a view to evaluating the optimal site for performing a radio frequency ablation, should also be considered as further support for the hypothesis of the effect of a reduction in the propagation rate and of an extension of the refractory period. The following references should also be referred to in this connection:
xe2x80x9cSubthreshold conditioning stimuli prolong human ventricular refractorinessxe2x80x9d by J. R. Windle, W. M. Miles, D. P. Zipes, and E. N. Prystowsky, in Am. J. Cardiol. 1986; 57; pages 381-6, and
xe2x80x9cSubthreshold stimulation in the region of the slow pathway during atrioventricular node re-entrant tachycardia: correlation with effect of radio-frequency catheter ablationxe2x80x9d by S. Willems, C. Weiis, T. Hofmann, C. Rickers, and T. Meinertz in JACC Vol. 29, No. 2, February 1997, pages 408-15.
The object of the present invention is to provide improved solutions which can take advantage of the above-described phenomenon, known as electrotonic inhibition, to achieve an improvement in the course of the electrical heart-stimulation effect.
In one aspect, this invention is a heart-stimulation device comprising first stimulator means for applying electrical stimulation pulses at a first intensity level; second stimulator means for applying electrical stimulation pulses at a second intensity level; the first and second intensity levels being above and below a stimulation threshold, respectively; control means in electrical communication with the first and second stimulator means and capable of selectively controlling the generation of the stimulation pulses in the first and second stimulator means at the first intensity level and at the second intensity level. Preferably, the first stimulator means and the second stimulator means are configured to stimulate distinct regions of the heart. The first stimulator means may stimulate at least one of an atrial region and a ventricular region, and the second stimulator means may stimulate the heart muscle in the region of the atrioventricular node. The second stimulator means may further comprise sensing means for sensing the activity of the heart muscle. The control means can vary selectively at least one intensity parameter selected from the group of pulse amplitude, duration and frequency. The second stimulator means may be configured to generate a sub-threshold stimulation pulse in the form of individual pulses or repeated pulse trains and/or the pulse may synchronous or asynchronous with spontaneous heart activity.
In another aspect, this invention is an electrical heart-stimulation device comprising a first electrode adapted to apply electrical stimulation pulses at a first intensity level; a second electrode adapted to apply electrical stimulation pulses at a second intensity level; the first and second intensity levels being above and below a stimulation threshold, respectively; a processing unit in electrical communication with the first and second electrode, the processing unit capable of selectively controlling the generation of the electrical stimulation pulses in the first and second electrodes at the first intensity level and at the second intensity level.
In yet another aspect, this invention is a method of stimulating the heart, comprising providing a processing unit in electrical communication with a means for detecting atrial fibrillation and with first and second electrodes; implanting the electrodes in the heart; providing electrical stimulation pulses to the heart from the first electrode above a stimulation threshold; and providing electrical stimulation pulses to the heart from the second electrode below a stimulation threshold, wherein the electrical stimulation pulses are delivered in response to the detection of atrial fibrillation.